The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder, which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
An engine configured for spark ignition combustion can be adapted to operate in a stratified charge combustion mode under predetermined speed/load operating conditions. It is known that operating lean of stoichiometry using a stratified combustion charge can improve fuel economy but can increase exhaust emissions, including nitrogen oxides (NOx). It is known to use an ammonia-selective catalytic reduction device to reduce NOx in the presence of a reductant, e.g., urea. It is known that refilling a urea tank can burden an operator.
Known aftertreatment systems for internal combustion engines operating lean of stoichiometry can include a three-way catalytic converter followed by other exhaust aftertreatment devices, including a lean-NOx reduction catalyst, also referred to as a lean NOx adsorber and a selective catalytic reduction (SCR) catalytic device. Known three-way catalytic converters function to oxidize engine-out hydrocarbon (HC), carbon monoxide (CO), and reduce nitrides of oxygen (NOx) emissions during stoichiometric engine operation and oxidize HC and CO emissions during lean operation.
Known SCR devices include catalyst material(s) that promotes the reaction of NOx with a reductant, such as ammonia (NH3) or urea, to produce nitrogen and water. Reductant materials may be injected into an exhaust gas feedstream upstream of the SCR device, requiring injection systems, tanks and control schemes.
Known catalyst materials used in SCR devices include vanadium (V) and tungsten (W) on titanium oxides (TiO2). Mobile applications include base metals including iron (Fe) or copper (Cu) with a zeolite washcoat as catalyst materials. Material concerns for catalyst materials include temperature operating ranges, thermal durability, and reductant storage efficiency. For mobile applications, SCR devices have an operating temperature range between 150° C. and 600° C., which may vary depending on the selected catalyst material(s) and operating conditions.